Now that I think of it, Betax #5s aren't that expensive and they're huge.

In fact, I do this with an Ilex 5. Rather than trying to find the shutter by itself, look for a nice lens that you could use already in a big shutter (mine is a 10" WF Ektar), and then adapt from there.

Also, some of the earlier Sinar shutters have come down in price and can be adapted easily to many 8x10" cameras.